1. Field of the Invention
This invention relates to photoconductors and more specifically to photoconductors used for coating a substrate for use in electrophotographic copy machines and computer printers utilizing infrared light and to a method for making these photoconductors.
2. Description of the Prior Art
Two general types of electrophotographic processes for producing copies of information (whether stored in computer memory or on paper) are widely used. The first method is known as the "charge transfer process" and the second is known as the "toned image transfer process".
In the charge transfer process, an electric charge is put on the surface of a cylindrical drum, the surface of which is coated with a photoconductive material. Selected regions on the surface of the charged drum, corresponding to those regions which are to appear untoned or white on the printed copy, have the charge stored thereon erased by the application of light, thus causing stored charge to be discharged to the underlying conductive drum. A dielectric paper is then placed on the drum, and the remaining charge is transferred from the surface of the drum to the dielectric paper. With selected regions on the paper now charged, a toner is applied to the paper, and adheres to those regions on the paper which are charged. The toner is then "fixed" (i.e., made permanent, usually by heating), thus providing a reproduced image on the dielectric paper corresponding to the pattern of charge on the surface of the drum.
In the toned image transfer process, the surface of a photoconductive drum is first charged with an electric potential, and selected regions of this surface, corresponding to those regions of the printed copy which are to appear untoned or white, have the electrical charge removed by the application of light, thus causing stored charge to be discharged to the underlying conductive drum. Toner is then directly applied to the drum, and adheres to those portions of the drum which remain charged. Plain paper is then placed on the drum, with the toned image formed on the drum being transferred to the paper. The toned image on the paper is then fixed, resulting in an electrophotographic copy remaining on the paper.
Photoconductors used on the surface of drums in such typical copying apparatus are required to reproduce "red color information" (i.e. red color on an original) in order that red information will appear on the copy as black or a shade of gray. Therefore, prior art photoconductors are designed to have relatively low sensitivity (the rate of decay of stored charge for a given intensity of light) at the red end of the visible spectrum, as well as in the infrared region, in order that the red portion of the spectrum will not affect the photoconductor, as does white light. In contrast, for use in laser printers, it is necessary to utilize a photoconductor for coating the surface of the drum which has enhanced sensitivity in the red and infrared (IR) regions. It is particularly desirable to utilize a photoconductor which is sensitive in the near infrared region within the range of approximately 750-850 nanometers in conjunction with laser diodes emitting infrared light within this range. In this manner, the infrared light emitted from the infrared laser diodes is used to selectively remove charge from selected regions of the coated surface of the photoconductor drum.
Photoconductors are well known in prior art. In particular there are several patents and publications describing the preparation of cadmium sulfide/cadmium carbonate in photoconductors, including U.S. Pat. Nos. 3,494,789; 3,975,306; and 3,704,123. However, the most closely related prior art is application Ser. No. 204,966 filed by Feinleib and assigned to Varian, Inc., which is hereby incorporated by reference into this application.
Cadmium sulfide based photoconductors have a very high dark conductivity, thus making them difficult to use. "Dark conductivity" is the conductivity of the photoconductor when not exposed to light. A high dark conductivity causes a high dark decay wherein charges stored on a dark (i.e., not exposed to light) photoconductive surface dissipate or decay rapidly, thus preventing proper operation of the printing machine. Furthermore, the use of cadmium sulfide in a binder is not practical for use in Xerographic copy machines because the resulting photoconductive material has a memory, which retains previous electric charge, thus causing "print-through" of a previous copy. Electrically neutralizing the charge on the photoconductive drum is generally not effective in eliminating the memory of cadmium sulfide photoconductors. Furthermore, cadmium sulfide photoconductors must be used in extremely thin layers because cadmium sulfide absorbs light to only shallow depths. With the use of very thin layers of cadmium sulfide, insufficient charge is sustained on the surface of the photoconductor, thus preventing good imaging on the resulting copy. In thicker layers of cadmium sulfide based photoconductors, an insufficient amount of the charge stored on the surface of the photoconductor is removed by the application of light to provide good imaging on the resulting copy.
It is known in the prior art that the sensitivity of cadmium sulfide based photoconductors in the red region (both visible red and infrared) can be increased by increasing the concentration of selected dopants within the photoconducting material (such dopants are typically copper and chloride) and by replacing a portion of the cadmium sulfide with cadmium selenide. However, both of these prior art measures of increasing the red sensitivity of cadmium sulfide based photoconductors will increase the photoconductor's dark conductivity, with a resultant increase in the photoconductor's dark decay, thus making the photoconductor unsuitable for use in copy machines.
Another prior art photoconductor utilizes a cadmium sulfide/cadmium carbonate base which also contains a small amount of cadmium oxide. Such a photoconductor is described in co-pending U.S. patent application Ser. No. 204,966. The photoconductive material disclosed in this co-pending application overcomes certain limitations of prior art cadmium sulfide/cadmium carbonate photoconductors and comprised a cadmium sulfide/cadmium carbonate type photoconductor with increased charge acceptance, reduced memory effects, more dependable performance in high humidity environments, and increased stability of speed over long periods of time as compared to prior art cadmium sulfide based photoconductors. The photoconductor of the abovementioned co-pending patent application has permitted the use of highly photosensitive cadmium sulfide as an electrophotographic medium, due to its ability to absorb light to a good depth (i.e., approximately 20-35 microns), thereby allowing layers of photoconductive material to be formed on a drum surface to a sufficient thickness to sustain sufficient surface charge on the photoconductor after charging, thus resulting in a useful electrophotographic photoconductor. In addition, such a photoconductor allows "memory effects" (i.e., retained previous electric charge causing "print through" of a previous copy) to be eliminated by proper machine designs, which includes pre-exposing the photoconductor to illumination thereby discharging all electric charge retained by the photoconductor. Of importance, the mere electrical neutralization of surface charges on the photoconductor drum is insufficient to completely eliminate this phenomenon. Unfortunately, the infrared sensitivity of the photoconductor disclosed in the co-pending application is quite low, but is somewhat increased by the methods described above, (increased dopants and the use of cadmium selenide) subject to increased dark conductivity.
Still another prior art photoconductor utilizes a cadmium selenide base. The cadmium selenide photoconductor is more sensitive to infrared light than cadmium sulfide photoconductors, yet has a high dark conductivity, thus making it relatively ineffective for use in copy machines.
Other prior art photoconductors which are sensitive in the infrared region include arsenic triselenide and tellurium doped arsenic triselenide. However, each of these photoconductors has high dark conductivity, great temperature sensitivity, and is inherently unstable, thus making them difficult to use in copy machines. In addition, these photoconductors are difficult to fabricate and can only be used in one charging mode (i.e., they are not bipolar). A bipolar photoconductor is one which may be charged alternatively either positively or negatively, and the charge may be removed at selected regions on the photoconductor surface by the application of light.
Thus, to overcome these limitations of prior art photoconductors, it is desired to provide a photoconductor which is highly sensitive in the infrared region, and yet does not exhibit excessively high dark conductivity.